Alex, in terms of your second question first: we each of us carry our lives into the dojo with us, good and bad, injuries and meds, limitations, frustrations, goals we are explicitly working on, buttons we don't know we have until they get pushed.....the sole concern regarding any ingested substance is, does it pose a hazard to myself or my training partner? Standard antidepressants currently in use certainly won't affect your partners' safety and shouldn't affect your's except that some have anticholinergic activity (drying) which may make you need to be more aware of coming to class well hydrated and, if it's a long class like at a seminar or during very hot weather bringing liquid along.

In terms of effects on depression - as you note, in itself aikido is not a treatment modality - however, most studies do show that physical activity has a beneficial effect. There is also a lot to be said for being able to get yourself to the dojo and on the mat even if depressed - much the way folks say that having to get up to feed a cat or walk the dog is often a lifesaver - the ability to march oneself through a daily routine even on the day it brings no joy is beneficial in itself in that the routine is there, maintained, the day it begins to bring joy again.